Powder metallurgy provides a useful and versatile technique for making non-ferrous metal parts, particularly those having an irregular shape. Briefly, powder metallurgy involves forming in a die having the precise shape of the desired part, compacting the selected non-ferrous metal or metal alloy powder, usually copper, brass or bronze, and then sintering the compacted part at an elevated temperature under a gaseous atmosphere which protects the part from oxidation during the extreme heat of the sintering process.
Although the parts so made conform closely to the shape of the die, they very often require further machining to obtain the desired dimensions and surface finish. In addition, many structural metal parts made by powder metallurgy undergo drilling, tapping, boring and sinking operations for attachment of fasteners and other parts. Thus, even though powder metallurgy can significantly reduce the amount of machining and cutting which a part must undergo, machinability remains an important property of sintered powder metallurgical parts.
Machinability in this context may be understood as the relative ease with which a portion of a part may be removed under specific cutting conditions, for example by drilling. Machinability of a metal part depends upon many factors, such as density, grain and pore size, alloy or metal composition, and microstructure of the sintered metal compact. Of these factors, chemical composition seems to most affect the machinability of the workpiece.
In the past, wrought and powder metallurgical products have been made more machinable by adding lead, tellurium and sulfur to the liquid copper, brass or bronze metal from which the metal powder will be formed. For example, C14500 tellurium cooper contains 0.50% tellurium, while C14700 sulfur copper contains 0.35% sulfur; such additives provide improved machinability. C33500, C34000 and C34200 designate low, medium and high lead content brasses (containing 0.5%, 1% and 2% lead respectively).
The use of lead as an additive in brass parts, however, has recently become viewed as undesirable. When atomizing the molten metal alloy, lead vapors may contaminate the air, and lead can be absorbed by workers handling lead powders during parts fabrication. High blood levels of lead have been implicated in a variety of health maladies, and the leaded content of products should be reduced or eliminated where possible.
Therefore, a need exists for a brass alloy having no lead content, but nevertheless having good machinability characteristics. Various additives for brass powders have been tried for different purposes, with varying results. For example, U.S. Pat. No. 4,656,002 (Miyafuji) discusses adding in excess of 0.001 wt % magnesium to a copper alloy to form a eutectic compound in the presence of sulfur. The resulting alloy is said to have increased hot working properties.
In U.S. Pat. No. 4,851,191, the patentees state that they improve copper alloys intended for high speed and heavy load applications by forming a fine grain Mn-Si precipitate with added tin and boron.
None of the foregoing additives seems suited to improving the machinability of copper alloys such as brass. Therefore, a principal object of the present invention is to provide a sinterable copper alloy having improved machinability characteristics.